I suppose you mean 23x for x = 2.55
This will be 23(2.55) = 58.65
For young children, this is incredibly advance stuff.
Of course, for us it looks freaking simple.
What I will suggest you do is to NOT start from decimals.
Instead teach your child using counters or coins to show him the mupltication of whole numbers as addition instead.
Decimals should come in at a much later stage, and you should get some place value chart and counters to teach your child basic decimal addtion before moving on to such multiplication.